As is known, a solid-propellant engine comprises a casing filled with solid propellant, a device for ignition of the propellant, and a terminal nozzle, through which the propellant, by burning, generates a desired propulsive thrust. The casing comprises a core made of steel or of a composite material, which has a cylindrical tubular portion and a closing portion shaped like a spherical cap and is insulated by being coated both internally and externally with a layer of specific thermally insulating material to define an adequate thermal protection of the core.
Normally, the layer of thermally resistant material is obtained by forming separately from the core two shells or caps, which, once the core has been formed in vacuum conditions, are, one, inserted within the core bringing it into contact with the internal surface of the core itself and, the other, fitted on the core causing it to adhere to its outer surface. Once the coupling is terminated, the caps are vulcanized in autoclave to complete the coating.
The known modality of insulation just described requires three dedicated apparatuses that are different from one another, two for making the caps and the third for assembling the caps themselves on the core. For these reasons, the mode of insulation described involves extremely long times and high costs of implementation, there being necessary three distinct working steps, rendered, on the other hand, even more complex by the particular geometry of the casing. However accurate the operations of production of the shells or caps may be, these frequently have defects and dimensional instability, randomly distributed porosities, and for this reason induce positioning errors or faults.
Even though the maximum care is then taken in positioning and in the subsequent operation of vulcanizing, between the caps and the core there are frequently present areas in which they are not properly stuck together, which, along with the aforesaid porosities, jeopardize sensibly the efficiency of the insulation and consequently the reliability of the casing.